Wireless communication systems allow remote and often portable transceiving devices, e.g., radio telephones ("mobile units"), to communicate bi-directionally with each other and with stationary transceiving devices, e.g., cellular stations ("fixed stations") over great distances. FIG. 1 shows a typical wireless network 10, in which several mobile units 12, 14, 16 communicate with two fixed stations 18, 20. The fixed stations 18, 20 communicate with each other, e.g., via radio frequency (RF) signals 22 or via a public switching telephone network 24 (PSTN).
Many wireless networks, including cellular networks such as the Personal Handyphone System (PHS) networks in Japan and the Digital European Cordless Telephone (DECT) networks in Europe, utilize dynamic channel assignment, through which fixed stations with overlapping coverage areas use and reuse preassigned carrier frequencies. In some systems, such as PHS, the mobile and fixed units employ time division multiple access (TDMA) and/or time division duplex (TDD) communication, which allows each fixed station to communicate with multiple mobile units in a given transmit/receive time period. Each transmit/receive period may include several discrete time slots, each dedicated to transmitting information between a fixed station and a particular mobile unit. A transceiver unit may be assigned to transmit at any given carrier frequency during each time slot, but tends to remain assigned to that slot for a relatively long period of time.
The fixed stations in many TDMA/TDD-based networks, including PHS networks, may be either public or private. In general, a public fixed station may be accessed by any mobile unit within its range, and a private fixed station may be accessed only by phones registered to it. While public fixed stations typically operate under the control of synchronized internal clocks, the clocks among private fixed stations are not synchronized in practice. Moreover, the clocks in private PHS fixed stations must be accurate only to 5 ppm, so over time the clocks in private fixed stations tend to drift with respect to one another. Because a virtually limitless number of PHS mobile units and fixed stations may exist within a given geographic area, and because PHS networks use Dynamic Channel Assignment, private fixed stations are subject to great variations in the interference they experience from other fixed stations. For example, a carrier frequency that presents relatively little interference when first selected by a fixed station may become too noisy for adequate communication when another fixed station suddenly switches to the carrier or as the transmit/receive periods of the fixed stations gradually drift onto each other.